In directional or controlled trajectory drilling, the S vertical inclination and azimuth of a drilled bore may be controlled such that the bore may extend from the surface to a target area which is not vertically aligned with the point on the surface where drilling commences. This permits a wide area to be accessed from a single drilling location and is therefore particularly useful in offshore drilling operations.
Rotation of the drill bit mounted on the lower end of the drill string is achieved by rotation of the entire drill string, by a turntable on the surface, and often also by a downhole motor located on the drill string adjacent the bit. The downhole motor is usually driven by the drilling fluid which is pumped through the string. Steerable downhole motors include a "bent" housing or elbow which introduces a small deviation (around 1.degree.) in the end portion of the drill string. When the entire string is rotating such an elbow has little or no effect on the bore trajectory. However, if the string is stopped and then adjusted such that the motor bend is in a desired direction, rotating the drill bit using only the downhole motor will result in the trajectory of the well deviating. Drilling in this manner without rotation of the drill string may be very time consuming as static friction between the non-rotating parts of the string and the bore wall tends to produce a stick-slip progression of the string through the bore. This results in sudden increases in the weight (downward force) being applied to the bit and motor, causing the motor co stall. The drill string must then be picked off bottom before drilling may restart. This problem may even result in it becoming impossible to drill any further without rotating the drill string and is particularly acute in horizontal and extended reach wells.
Attempts have been made to provide drilling apparatus which will permit bore trajectory to be varied or controlled while still rotating the drill string, primarily by providing a non-rotating eccentric mass on the drill string adjacent the drill bit, and which mass engages the "low" portion of the bore wall and supports the drill string. A radially extending blade is mounted on the mass and engages the bore to produce a lateral force on the drill string causing the drill bit to deviate from its existing path, or at least prevents further deviation in the direction of the blade. However, the success of such apparatus has been limited as the mass provides an unstable support for the heavy drill string, such that the mass is likely to topple and be moved to one side by the string, which will tend to move downwards to occupy the lower part of the bore. Examples of such arrangements are illustrated in U.S. Pat. Nos. 4,638,873 and 4,220,213.
Other forms of directional drilling apparatus for controlling hole direction or inclination by providing eccentric or offset blades or members are described in U.S. Pat. Nos. 3,062,303, 3,092,188, 3,650,338, 3,825,081 and 4,305,474.